Yoldia Sea

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Yoldia Sea is a name given by geologists to a variable brackish-water stage in the Baltic Sea basin that prevailed after the Baltic ice lake was drained to sea level during the Weichsel glaciation. Dates for the Yoldia sea are obtained mainly by radiocarbon dating material from ancient sediments and shore lines. They tend to vary by up to a thousand years, but a good estimate is 10,300 – 9500 radiocarbon years BP, equivalent to ca 11,700-10,700 calendar years BP. The sea came to an end when isostatic rise of Scandinavia closed or nearly closed its effluents, altering the balance between saline and fresh water. The Yoldia Sea became Ancylus Lake. The Yoldia Sea stage had three phases of which only the middle phase had brackish water.

The name of the sea is taken from the bivalve, Portlandia or Yoldia arctica, found around Stockholm. This bivalve requires cold saline water. It characterizes the initial phase of the Yoldia Sea, during which saline water poured into the Baltic, before the acceleration of glacial melting.

[edit] Description

The Baltic Ice Lake came to an end when it overflowed through central Sweden and drained, a process complete by about 10,300 BP (radiocarbon years). The straits through the present Stockholm region were the only outlet at that time. When lake level reached sea level the difference in salinity caused a backflow from the North Sea, creating saline regions in which Yoldia flourished. This phase lasted until about 10,000 BP.

Subsequently increased melting of the glacier provided additional fresh water and the lake became stratified, with salt water on the bottom and fresh on top. Over the life of the sea and from location to location the salinity was a variable. Whether it is possible to speak of stages of salinity that would apply uniformly to the whole sea is debatable.

Also at about 10,000 BP, the lake/sea broke through Denmark creating the first Great Belt. It was less than 1 km wide and included two channels at the northern end. It was blocked again by rising land that created Ancylus Lake.

Geographically, the Gulf of Bothnia remained under the ice. The Gulf of Finland was open but most of Finland was an archipelago, over which debris carried by glacial streams gradually spread. A land bridge joined Germany to southern Sweden through Denmark. Relieved of its weight of ice, Finland rose gradually and unevenly from the sea. Parts of the Yoldia shoreline are above sea level today while other parts remain below. The Yoldia Sea toward its end was about 30m below current sea level. A channel at the location of the Neva River connected Yoldia Sea to Lake Ladoga.

The Yoldia Sea existed entirely within the Boreal Blytt-Sernander period. The forests and species lining its shores were boreal. Mesolithic cultures continued to occupy Denmark/south Sweden and the southern shores of the sea. The sea as an ecologic system came to an end when Scandinavia rose sufficiently to block the flow through the Stockholm area and the saline balance shifted toward a lacustrine ecology once again.